Weather summaries said visibility on the ground was down to 300m around the time that the plane first tried to land, while broken cloud cover was down as low as 100ft above ground. The constant fog is due to the airport's location high above sea level.

However, Met Eireann points out the number of fight diversions due to foggy conditions has "dropped dramatically" since 1990 when the airport received 'Category 2 status.'

This means modern aircraft are capable of instrument landing in fog when visibility is restricted or non-existent.

Nevertheless, the airport, which opened in October 1961, had an exemplary safety record until yesterday's tragedy.

The foggy conditions are known to pilots, and the airport is equipped with state-of-the-art equipment.

Dense fog remains, however, a pilot's nightmare, and has been implicated in many airliner disasters worldwide.

Pilots generally prefer to be able to see the runway as they land. If they cannot, they normally veer away and move to a holding position until the fog clears.

If the fog persists they normally divert to the nearest airport. When visibility is impaired, and with fuel running low, aircraft can switch to the instrument landing system to land, where they do not need to see the runway.

Fog has been the single biggest cause of many air tragedies.

In 1999, a crash which killed 24 people, including an Irish aid worker, in Kosovo, was blamed on dense fog as the plane tried to land in Pristina Airport. And last April, fog was responsible for a crash in Russia which killed 96 Polish officials, including president Lech Kaczynski and his wife.